This invention relates to a process for the selective solvent extraction of hydrocarbon mixtures, for example lubricating oils, and in particular, a process for the solvent extraction of lubricating oils using furfural. The invention also relates to the processing of residual petroleum stocks by visbreaking in the presence of certain highly aromatic hydrogen-donor materials obtained from a solvent extraction process such as one employing furfural.
Solvent extraction is a well known process for the separation of aromatic hydrocarbons from mixtures with nonaromatic hydrocarbons of similar boiling range. Furfural has been found to be an excellent selective solvent in the separation of hydrocarbons of relatively high boiling point, for example, hydrocarbons useful in the manufacture of lubricating oils and catalytic cracking feed stocks. Furfural extraction has also been found useful in the manufacture of kerosine and low boiling gas oil products where a raffinate of low aromatic content is produced.
In a typical furfural extraction process, a liquid hydrocarbon mixture containing aromatic and non-aromatic hydrocarbons is contacted with liquid furfural in an extraction column effecting formation of a raffinate phase, which is withdrawn from the top of the column and which contains a major portion of the non-aromatic hydrocarbons, and an extract phase which is withdrawn from the bottom of the column and which contains most of the furfural containing dissolved hydrocarbons including a major portion of the aromatic hydrocarbons and the remaining non-aromatic hydrocarbons. The two phases can then be separated into their constituents by distillation. According to U.S. Pat. No. 3,205,167, it is also known to treat the extract phase by cooling thus separating it into one phase comprising a naphthenic oil known as a pseudo raffinate and containing a little solvent and the other phase comprising the so-called "extract proper" containing the more aromatic and sulfurized components of the oil and a large quantity of solvent. U.S. Pat. No. 3,205,167 is silent regarding the specific temperature range within which this cooling operation is to be carried out and says nothing of the composition or properties of the "extract proper".
Visbreaking, or viscosity breaking, is a well known petroleum refining process in which reduced crudes are pyrolyzed, or cracked, under comparatively mild conditions to provide products having lower viscosities and pour points, thus reducing the amounts of less viscous and more valuable blending oils, so-called "cutter stock", required to make the residual stocks useful as fuel oils. The visbreaker feed stock usually consists of a mixture of two or more refinery streams derived from sources such as atmospheric residuum, vacuum residuum, furfural-extract, propane-deasphalted tar and catalytic cracker bottoms. Most of these feed stock components, except the heavy aromatic oils, behave independently in the visbreaking operation. Consequently, the severity of the operation for a mixed feed is limited greatly by the least desirable (highest coke-forming) components. In a typical visbreaking process, the crude or resin feed is passed through a heater and heated to about 425.degree. to about 525.degree. C. at about 450 to about 7000 kPa. Light gas-oil may be recycled to lower the temperature of the effluent to about 260.degree. to about 370.degree. F. Cracked products from the reaction are flash distilled with the vapor overhead being fractionated into a light distillate overhead product, for example, gasoline and light gas-oil bottoms, and the liquid bottoms are vacuum fractionated into heavy gas-oil distillate and residual tar. Examples of such visbreaking methods are described in Beuther et al., "Thermal Visbreaking of Heavy Residues," The Oil and Gas Journal, 57: 46, Nov. 9, 1959, pp. 151-157; Rhoe et al., "Visbreaking: A Flexible Process," Hydrocarbon Processing, January 1979, pp. 131-136; and U.S. Pat. No. 4,233,138.
European Patent Application 0 133 774 describes a process for visbreaking a heavy petroleum residual oil which comprises subjecting the oil to an elevated temperature for a period of time corresponding to an equivalent reaction time of 250 to 1500 ERT seconds at 427.degree. C., in the presence of from 0.1 to 50 weight percent, based on the residual oil, of a hydro-aromatic solvent having a content of H.sub.Ar hydrogen (protons which are attached directly to aromatic rings and which constitute a measure of aromaticity of a material) and H.sub.alpha hydrogen (protons which are attached to non-aromatic carbon atoms themselves attached directly to an aromatic ring, e.g., alkyl groups and naphthenic ring structures) each of at least 20 percent of the total hydrogen content, and recovering a fuel oil product having a viscosity lower than that of the starting residual oil. The hydro-aromatic solvent used in this process is a thermally stable, polycyclic, aromatic/hydroaromatic distillate hydrogen donor material, preferably one which results from one or more petroleum refining operations. The hydrogen-donor solvent nominally has an average boiling point of 200.degree. to 500.degree. C., and a density of 0.85 to 1.1 g/cc. Examples of such suitable hydrogen donor materials are highly aromatic petroleum refinery streams, such as fluidized catalytic cracker "main column" bottoms which are highly preferred, fluidized catalytic cracker "light cycle oil," and thermofor catalytic cracker "syntower" bottoms, all of which contain a substantial proportion of polycyclic aromatic hydrocarbon constituents such as naphthalene, dimethylnaphthalene, anthracene, phenanthrene, fluorene, chrysene, pyrene, perylene, diphenyl, benzothiophene, tetralin and dihydronaphthalene, for example. Such refractory petroleum materials are resistant to conversion to lighter (lower molecular weight) products by conventional non-hydrogenative procedures. Typically, these petroleum refinery residual and recycle fractions are hydrocarbonaceous mixtures having an average carbon to hydrogen ratio above about 1:1, and an average boiling point above 230.degree. C.